There are many radio access technologies which enable a mobile terminal to communicate packet data to and from data networks. Such technologies include radio access interfaces based on the IEEE 802.11 standard for WLAN (Wireless Local Area Network) and the 3GPP defined General Packet Radio Service (GPRS). A mobile terminal communicates data to and from a data network, for example the internet, via a WLAN by establishing a radio link with a nearby WLAN access point. Typically, a WLAN router connected to the access point will provide the mobile terminal with a temporary internet protocol (IP) address which can then be used by the mobile terminal as an address from which to transmit and receive packet data to and from the network. Similarly, in order for a mobile terminal to communicate data to and from a GPRS data network, for example the internet, the mobile terminal establishes a radio connection via a nearby base station with the core network of a mobile services provider. In order for the mobile terminal to communicate packet data with an external network a GGSN (GPRS Gateway Support Node) present within the core network allocates the mobile terminal a temporary IP address. The GGSN then acts as a gateway between the external network and mobile terminal: the GGSN forwards packets transmitted from the mobile terminal on to their destination on the external network and receives packets addressed to the temporary IP address allocated to the mobile terminal and forwards these received packets to the mobile terminal.
In addition to the various radio technologies that enable a mobile terminal to communicate packet data, a number of mobile IP protocols have been defined. These mobile IP protocols include functionality to enable a user to maintain a secure active IP session on a network whilst moving across access networks. This is achieved principally by employing an IP tunnel in which packets are “tunnelled” from one node to another.
Given the variation between different radio access technologies and the complexity inherent in mobile IP protocols, it is difficult to provide a system which allows a mobile terminal to move between networks accessed through different radio access technologies and using a mobile IP protocol, whilst maintaining efficient communication of packet data.